In a wireless local area network (WLAN), an access point (AP) may communicate with one or more mobile stations (STA) over a wireless communication link. In non-secure systems, signals transmitted by the AP may be detected by any station within range of the AP, including outside listening parties and unauthorized users. This raises a concern for secure WLAN communication.
For example, in a single-antenna WLAN system, the AP may transmit a signal at its maximum power output to increase network coverage and range of the communication link. The transmitted signal energy will propagate in an isotropic pattern and may be detected anywhere within the reach radius of the AP, regardless of the location of the intended recipient. Thus, transmissions in a single-antenna WLAN system may be easily intercepted by parties other than the intended recipient.
As is known in the art, beamforming may allow a collection of omni-directional simple antennas to act like a single, highly focused, directional antenna. Each transmit antenna may transmit the intended signal multiplied by a certain weight, and by dynamically controlling the weights of each antenna the transmission may be directed to a desired location. While each antenna transmits an electromagnetic wave that propagates isotropically, the assignment of different amplitudes and phases to simultaneous transmissions from the multiple antennas can create a wave interference pattern which combines coherently at a desired location to reproduce the intended signal.
Spatial division multiple access (SDMA), also known as spatial-division multiplex (SDM), is a method for multiplexing several signal streams, each one targeted to a different destination, simultaneously, by utilizing multiple antennas. A SDMA channel access method may enable the use of the same frequency at the same time in different spaces. For example, multiple formed beams may be transmitted to several remote stations simultaneously. Typically, a plurality of orthogonal beams may be formed such that power directed towards intended destination stations is maximized, while the interference generated to other stations may be minimized.
In a multiple-antenna communication system, beamforming may be used to increase the link budget of a communication link by directing the emitted energy towards the location of an intended recipient. Thus, a potential interceptor, having a spatial signature different from that of the intended recipient, may receive a lower energy signal, resulting in a somewhat diminished signal-to-noise ratio (SNR). However, the interceptor may still be able to decode the signal.
In a SDMA system, an interceptor, having a different spatial signature than an intended recipient, may receive additional interference caused by superposition of the other station signals, resulting in a degraded signal-to-noise ratio (SNR) due to both decreased energy and increased noise. Still, there are many situations in which this SNR degradation is limited, for example, when the number of intended recipients, K, is significantly smaller than the number of transmitting antennas, thereby undesirably improving the chances of the interceptor to detect the SDMA signal.